1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type such as a copying machine, a printer and the like, and a corona charging apparatus used in such an image forming apparatus.
2. Related Background Art
FIG. 4 is a schematic constructural view showing an example of a conventional image forming apparatus using a corona charging apparatus (scorotron charging apparatus) as charging means.
The image forming apparatus comprises a drum-shaped electrophotographic photosensitive body (referred to as xe2x80x9cphotosensitive drumxe2x80x9d hereinafter) 1 as a body to be charged (image bearing member), a scorotron charger (charging means) 2, an exposure device 3, a developing device 4, a corona transfer charger (transfer charger) 5, an electrostatic separating charger (separation charger) 6, a cleaning device 7, a pre-exposure lamp 8 and a surface potential sensor 31, which elements 2 to 8 and 31 are disposed around the photosensitive drum.
In this conventional case, the photosensitive drum 1 is an A-Si (amorphous silicon) photosensitive drum rotated at a predetermined peripheral speed (process speed) in a direction shown by the arrow R1.
The scorotron charger 2 has a grid electrode 2a and a discharge wire electrode 2b which are housed in a shield case 2c having an opening and serves to charge a surface of the photosensitive drum 1 by corona ions generated by corona discharging.
The exposure device 3 serves to apply image exposure L corresponding to image information inputted from an image scanner portion 10 to the surface of the photosensitive drum 1 charged by the scorotron charger 2. The exposure device 3 has a laser driver 19, a laser diode 20, a polygon mirror 21 and a reflection mirror 22. A laser beam (intensity-)modulated in correspondence to a time series electrical digital image signal corresponding to the image information inputted from the image scanner portion 10 through an image processing portion 18 is outputted from the laser diode 20, and the laser beam is scanned by the polygon mirror 21 rotated at a high speed. Then, by applying the image exposure L, through the reflection mirror 22, to a photo-conductive layer (not shown) on the surface of the photosensitive drum 1, an electrostatic latent image corresponding to the inputted image information is formed.
The image scanner portion 10 serves to read an original 12 rested on an original glass plate 11 by scanning the original by an illumination lamp 13 and to convert the image information into an electric signal by a photoelectric converting element (CCD) 16. Light reflected from the original 12 scanned by the illumination lamp 13 is introduced into mirrors 14a, 14b, 14c and is focused on the photoelectric converting element 16 by a focusing lens 15. After the electric signal form the photoelectric converting element 16 is digitalized by an A/D converter 17, the signal is converted into image signals having 256 gradients from 00 (00 hex) to 255 (FF hex) proportional to image density, which is in turn outputted to the laser driver 19 through the image processing portion 18.
The developing device 4 serves to visualize the electrostatic latent image as a toner image by adhering toner to the latent image at a developing position.
Next, an image forming operation of the image forming apparatus will be explained.
During image formation, the photosensitive drum 1 is rotated at the predetermined process speed in the direction shown by the arrow R1 by driving means (not shown), and the surface of the photosensitive drum is uniformly charged by the scorotron charger 2. Then, the image exposure L from the exposure device 3 is applied to the charged surface of the photosensitive drum 1, thereby forming the electrostatic latent image corresponding to the image information of the original 12. The toner is adhered to the latent image by a developing sleeve 4a to which developing bias having the same (negative) polarity as charging polarity of the photosensitive drum 1 is applied, the latent image is developed as the toner image.
When the toner image on the photosensitive drum 1 reaches a transfer nip portion between the photosensitive drum 1 and the corona transfer charger (transfer charger) 5, the toner image on the photosensitive drum 1 is transferred onto a transfer material P such as a paper sheet (fed from a sheet feeding cassette (not shown) to the transfer nip portion at a predetermined timing) by the corona transfer charger 5 to which transfer bias having (positive) polarity opposite to the polarity of the toner is applied. The transfer material P to which the toner image was transferred is separated from the photosensitive drum 1 by the electrostatic separating charger (separation charger) 6 and the separated transfer material is conveyed between a fixing roller 23a and a pressure roller 23b of the fixing device 23, where the toner image is thermally fixed to the transfer material by heat and pressure from the fixing roller 23a and the pressure roller 23b. Thereafter, the transfer material is discharged out of the image forming apparatus.
On the other hand, after the transferring, transfer residual toner remaining on the surface of the photosensitive drum 1 is removed and collected by the cleaning device 7. Further, residual charges on the surface of the photosensitive drum 1 is removed by the pre-exposure lamp 8, thereby preparing for a next image formation.
By the way, due to dispersion in manufacture of the photosensitive drums 1, there is also dispersion in charging abilities of the photosensitive drums. Further, the charging ability is charged by a change in the discharging property of the scorotron charger 2 and change in charging property of the photosensitive drum 1 due to long term use and change in environment in which the image forming apparatus is used.
Thus, in the above-mentioned image forming apparatus, in order to absorb the dispersion in charging ability of the photosensitive drum 1, the surface potential of the photosensitive drum 1 was measured by the surface potential sensor 31, and grid voltage applied from a grid power supply 30 to the grid electrode 2a of the scorotron charger 2 was changed on the basis of measurement information from the surface potential sensor 31 so that the surface potential of the photosensitive drum 1 was maintained to the desired potential.
Further, as is in the above-mentioned image forming apparatus, when the photosensitive drum 1 having the small charging ability such as the A-Si (amorphous silicon) photosensitive body is used, voltage is applied to the shield case 2c of the scorotron charger 2 to increase the charging efficiency. In order to simplify the construction of the scorotron charger 2, electrical communication is made so that the potential of the grid electrode 2a becomes the same as the potential of the shield case 2c. 
By the way, in the above-mentioned image forming apparatus, the scorotron charger 2 is detachably attachable to the main body (not shown) of the image forming apparatus so that the charger can easily be exchanged and cleaned.
Further, the grid power supply 30 for applying the desired voltage to the grid electrode 2a and the shield case 2c of the scorotron charger 2 is provided in the main body (not shown) of the image forming apparatus, and there are a plurality of connectors and contacts in an electrical path from the grid power supply 30 to the grid electrode 2a and the shield case 2c of the scorotron charger 2.
In a condition that poor connection between the connectors or poor electric contact between the contacts occurs due to long term use and change in environment in which the image forming apparatus is used, i.e., in a condition that the supplying of electric power to the grid electrode 2a and the shield case 2c is interrupted, if the photosensitive drum 1 is charged by the scorotron charger 2, the charges generated by the corona discharging cannot flow into the grid electrode 2a and the shield case 2c. 
As a result, many charges flow into the photosensitive drum 1, thereby damaging the photosensitive drum 1. Particularly, when the photosensitive drum 1 is the A-Si (amorphous silicon) photosensitive body, since the voltage resistance is small, the photosensitive layer may be destroyed to make the use of the photosensitive drum impossible.
An object of the present invention is to provide a corona charging apparatus and an image forming apparatus, in which, even if poor electrical connection occurs between a grid electrode and a power supply, a body to be charged is prevented from being damaged.
Another object of the present invention is to provide an image forming apparatus, in which, even if poor electrical connection occurs between a grid electrode and a power supply, surface potential of an image bearing member charged by a corona charging apparatus can be maintained to a desired range.
A further object of the present invention is to provide a corona charging apparatus and an image forming apparatus, in which, even if poor electrical connection occurs between a grid electrode and a power supply, current can flow into the grid electrode.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.